The invention is related to the field of spectroscopy, and in particular to a direct and quantitative broadband absorptance spectroscopy technique using multilayer cantilever probes.
Fourier transform infrared (FTIR) and Fourier transform visible (FTVIS) spectroscopy, referred to as Fourier Transform Spectroscopy (FTS) in general, are well-known techniques to measure the optical properties of materials. By using different sample stages, FTIR and FTVIS systems can measure the transmittance and reflectance spectra of a sample in a single scan. Compared to dispersive spectrometers, FTIR and FTVIS have several important advantages. First, the multiplex advantage (or Fellgett advantage) implies that FTIR and FTVIS systems can simultaneously measure all wavelengths in the illumination source, thus a complete spectrum can be collected in a single scan faster than for conventional dispersive spectrometers. Second, the throughput advantage (or Jacquinot advantage) refers to the higher energy throughput of FTIR and FTVIS systems compared to dispersive spectrometers which results in a higher signal-to-noise ratio for the same spectral resolution. Third, since FTIR and FTVIS system use an interferometer to modulate the spectrum instead of prisms or gratings, stray light is negligible unlike in dispersive spectrometers.
Traditional methods to measure an absorption spectrum, including FTIR and FTVIS, are typically indirect, in the sense that the absorption spectrum of materials can only be calculated after measuring the transmittance and reflectance spectrum. This approach inevitably introduces uncertainties and errors in the result. For bulk materials, indirect methods are usually adequate to determine absorption characteristics, qualitatively or even quantitatively. For small samples on the micro or nanometer scale, however, it is difficult to use the FTS method to characterize the respective absorption spectrum, since the light scattered by the sample can cause disproportionally large errors.
An example of a direct absorptance measurement method is the photoacoustic method in which heat absorbed by the sample generates an acoustic wave that propagates in a surrounding gas. This acoustic wave is subsequently picked up by an acoustic detector. The photoacoustic method, however, is limited to macroscopic objects due to a lower signal-to-noise ratio for microscopic objects. Another commercial instrument for direct absorptance measurements is the Nano IR spectroscopy technique from Anasys Instruments, which detects absorbed power directly by measuring the thermal expansion using the tip of a micro-fabricated cantilever. This method is able to probe local areas equivalent to the size of the tip.
Micro-fabricated bilayer cantilevers have been used as direct and quantitative thermal sensors with an ultra-high power resolution. When the cantilever consists of multiple layers of materials with different thermal expansion coefficients, it bends under the influence of temperature change. The simplest case is a bilayer cantilever, which was used in the development of this platform. As photothermal sensors (or a heat flux sensors), micro-cantilevers have reported sensitivities as low as 4 pW. After carrying out appropriate calibration, the cantilevers can be used to extract quantitative data. Bilayer cantilevers have been used to measure the thermal conductance of thin films, near-field thermal radiation, the absorptivity of a thin gold film, the absorption of a few isolated chemical materials and the thermal conductivity of polyethylene nanofibers.